PubMed Health⌕ Search

Biomedical subjects

U Hagen

Publications and source records attributed to U Hagen.

18 recordsLinked to original sources
PubMed1992

Radical effects on mutation spectra in lambda phage.

Mutations in the lambda repressor gene cI (710 bp) were induced by 60Co-gamma radiation in dissolved lambda phage DNA. After in vitro DNA packaging to lambda phage particles (pack phage) and phenotypic expression of the mutants, DNA was sequenced directly. Two-thirds of mutations were located in the amino terminus region of the gene without any signs of hotspots. Changes consisted of (+1) insertions (25%) and base substitutions (75%). Transitions were exclusively G/C to A/T. Transversions were mostly G/C to C/G and few G/C to T/A. We did not find A/T to T/A transversions, A/T to G/C transitions, deletions and gross rearrangements. In most of the base substitutions a pre-existing base pair had been replaced by an A/T pair; this might come from 'non-instructional sites' like abasic sites. Several mechanisms for base substitutions are considered.

Bacteriophage lambda↗
PubMed1990

Molecular radiation biology: future aspects.

Future aspects of molecular radiation biology may be envisaged by looking for unsolved problems and ways to analyse them. Considering the endpoints of cellular radiation effects as cell inactivation, chromosome aberrations, mutation and transformation, the type of DNA damage in the irradiated cell and the mechanisms of DNA repair as excision repair, recombination repair and mutagenic repair are essential topics. At present, great efforts are made to identify, to clone and to sequence genes involved in the control of repair of DNA damage and to study their regulation. There are close relationships between DNA repair genes isolated from various organisms, which promises fast progress for the molecular analysis of repair processes in mammalian cells. More knowledge is necessary regarding the function of the gene products, i.e. enzymes and proteins involved in DNA repair. Effort should be made to analyse the enzymatic reactions, leading to an altered nucleotide sequence, encountered as a point mutation. Mislead mismatch repair and modulation of DNA polymerase might be possible mechanisms.

Animals↗
PubMed1990

Effect of irradiation and mutagenic chemicals on the generation of ADH2- and ADH4-constitutive mutants in yeast: the inducibility of Ty transposition by UV and ethyl methanesulfonate.

A strain defective in fermentation due to a deletion in the ADH1 gene was used to generate revertants which are constitutive mutants of the genes ADH2 and ADH4. By analyzing the DNA of the mutants we determined the frequency of Ty insertions into the promoter region of these genes. We found an increase in transposition after UV irradiation and treatment with ethyl methanesulfonate (EMS). Chemical inhibition of DNA synthesis and translation decreased the induced mutant yield and the transposition frequency, whereas inhibition of transcription had no effect. Differences in transposition frequencies between different strains and between the 2 loci lead to the conclusion that not only the transposable element itself but also the insertion sites determine the frequency of Ty transposition to a given locus.

Alcohol Oxidoreductases↗
PubMed1989

Biochemical aspects of radiation biology.

In order to analyze the mechanisms of biological radiation effects, the events after radiation energy absorption in irradiated organisms have to be studied by physico-chemical and biochemical methods. The radiation effects in vitro on biomolecules, especially DNA, are described, as well as their alterations in irradiated cells. Whereas in vitro, in aqueous solution, predominantly OH radicals are effective and lead to damage in single moieties of the DNA, in vivo the direct absorption of radiation energy leads to 'locally multiply-damaged sites', which produce DNA double-strand breaks and locally denatured regions. DNA damage will be repaired in irradiated cells. Error free repair leads to the original nucleotide sequence in the genome by excision or by recombination. "Error prone repair"(mutagenic repair), leads to mutation. However, the biochemistry of these processes, regulated by a number of genes, is poorly understood. In addition, more complex reactions, such as gene amplification and transposition of mobile gene elements, are responsible for mutation or malignant transformation.

Animals↗
PubMed1988

Action of gamma endonuclease on clustered lesions in irradiated DNA.

Irradiation of DNA in situ i.e. in phage particles or in the cell leads to alterations of single DNA nucleotides as well as to clustered lesions such as double strand breaks or unpaired DNA regions the latter being sensitive to digestion by S1 nuclease. A contribution will be made to the configuration of such S1-nuclease-sensitive sites (S1 sites). DNA from irradiated lambda phage containing S1 sites was treated with gamma endonuclease from M. luteus which is known to split the nucleotide strand at the position of oxidized pyrimidine base. It was found that the gamma endonuclease induces double-strand breaks at some of the S1 sites indicating double base damage within this site. However, half of the S1 sites are not converted into a double-strand break by the gamma endonuclease, indicating base damage only on one strand within the unpaired region.

Aspergillus oryzae↗
PubMed1988

[Intracellular recovery--basis of hyperfractionation].

The radiobiological basis of a hyperfractionated radiation therapy versus conventional fractionation with respect to therapeutic gain, i.e., improved normal tissue sparing for the same level of tumour cell inactivation, will be presented. Data on the recovery potential of various tissues as well as the kinetics of repair will be given. The problem of incomplete repair with short irradiation intervals will be discussed.

Animals↗
PubMed1979

Genomic integrity of T1 DNA after gamma-and ultraviolet irradiation.

T1 DNA, gamma-irradiated in the phage particle or irradiated with ultraviolet light was checked for structural integrity by kinetics of melting and reannealing. gamma-Irradiated DNA differed in all thermokinetic properties by a factor of 3-4 from DNA degraded by mechanical or enzymatical treatments. Ultraviolet irradiation caused much smaller effects than gamma-irradiation. Considering the frequency of pyrimidine dimers in relation to the gamma-ray induced lesions, strong evidence can be derived, that in addition to single base damages, local denatured regions are produced by gamma-irradiation. Such regions, formed possibly by direct absorption of radiation energy in DNA, i.e. by primary ionizations, are associated with base lesions and are passed over during reannealing.

Coliphages↗
PubMed1977

Action of intercalating agents on the activity of DNA polymerase I.

The effect of intercalating compounds such as 9-aminoacridine, quinacrine (atebrin), proflavine and daunomycin on the activity of DNA polymerase I(EC 2.7.7.7) was studied in vitro and compared with the binding of these acridines to native DNA. The enzyme kinetics were followed at various concentrations of DNA 3'-OH primer end groups and constant concentrations of deoxynucleosidetriphosphates as well as under the opposite conditions. The Km values for the DNA 3'-OH end groups were 16--38 nM and for the deoxynucleosidetriphosphates 2--5 micrometer, depending on the buffer and pH used in the enzymatic assay. All acridine derivates inhibit the DNA polymerase; at variable DNA concentrations a competitive inhibition was observed, where the Ki values ranged between 0.87 and 8.5 micrometer. At variable concentrations of deoxynucleosidetriphosphates and constant DNA concentration a non-competitive inhibition was observed. On denatured 3'-OH DNA as well as on poly(dA) - (dT)10 as substrate no inhibition by 9-aminoacridine was observed. 5'--3' exonuclease activity of DNA polymerase is inhibited by 9-aminoacridine but 3'--5' exonuclease activity on denatured DNA is not influenced by this intercalating compound. The affinity of the acridines to DNA was determined spectrophotometrically under conditions similar to those in the enzymatic assay and the computed frequency of intercalation was related to the inhibition of enzymatic activity. The mechanism of inhibition is explained by a disturbance of the structure of the double helical DNA due to the interaction of the bound acridine derivates.

Acridines↗
PubMed1977

Endonuclease activities in extracts of Micrococcus luteus that act on gemma-irradiated DNA.

Several protein fractions containing endonuclease activity against gemma-irradiated DNA (gamma-endonuclease) were isolated from M. luteus. The crude extract was eluted on a phosphocellulose column and chromatographed on TEAE cellulose and subsequently on hydroxyapatite. Five peaks of gamma-endonuclease were obtained from each preparation. Repeated experiments showed comparable chromatographic behavior of the fractions. There was no detectable activity of U.V.-endonuclease in the fractions with gamma-endonuclease but a small contamination of endonuclease against unirradiated DNA and against DNA with apurinic sites. The gamma-endonuclease is stimulated by, but is not dependent on, magnesium. Several tests for endonuclease activity have been used: the analysis of strand breaks in calf-thymus DNA or in PM2 DNA, and the determination of end-groups formed by endonuclease, either 3'OH end-groups or phosphomonoester end groups. From the results obtained it can be assumed that the strand breaks induced by the gamma-endonuclease carry 3'OH and 5' phosphate end groups.

Cobalt Radioisotopes↗
PubMed1976

In vitro repair of radiation-induced strand breaks in DNA.

DNA, gamma-irradiated in vitro or in isolated thymocytes was treated with several enzymes to achieve repair of the radiation-induced single strand braks. Whereas an incubation with polynucleotide ligase can join only 25% of the single strand breaks, a combined treatemnt with exonuclease III (EC 3.1.4.1), DNA polymerase I (EC 2.7.7.7), and polynucleotide ligase leads to repair of 80% of the breaks. For this in vitro repair the exonuclease III has to remove several, probably damaged, nucleotides from the 3'-terminal producing a single-stranded gap, which will be filled in by DNA polymerase I and joined by ligase. Tests for successful rejoining of the strand breaks were performed by showing the loss of primer 3'-OH sites for DNA polymerase I, by the resistance of incorporated nucleotides in the gap to removal by a second exonuclease III treatment, and by strand break determination in the analytical ultracentrifuge. 20% of the radiation-induced strand breaks will not be repaired by this combined treatment possibly due to an incomplete binding of the ligase on the 5'-terminals and/or an incomplete removal of the damaged 3'-terminals by exonuclease III.

Animals↗
PubMed1975

Changes in DNA secondary structure after gamma-irradiation.

Native calf thymus DNA was gamma-irradiated at 500 mug/ml in 0.01 M NaCl in the presence or absence of oxygen. By irradiation, an increasing amount of DNA becomes reactive with a water-soluble carbodiimide-derivative (CMEC). In the DNA sections reactive with CMEC the nucleotide strands are separated, a phenomenon previously described as radiation-induced denaturation. The dose-effect curve for the formation of denatured DNA shows an upward-bent form; a distinct oxygen effect of about 2 is observed. By a comparative study with DNA samples, degraded partially with DNAse I, it was shown that a minor part of the radiation-induced denaturation results from the formation of the radiation-induced single strand breaks, whereas the major part is a local denaturation independent of the strand breaks. In these locally denatured regions 20 to 50 nucleotide pairs are separated.

Animals↗
PubMed1975

Oxygen--effect on strand breaks and specific end-groups in DNA of irradiated thymocytes.

Thymocytes were irradiated with fast electrons up to 6 Mrad in the presence and absence of oxygen. The cells were treated before irradiation with a cold shock to prevent any repair rejection during irradiation. The DNA isolated subsequently was analysed for double-strand breaks (dsb), actual single-strand breaks (ssb) and alkali-induced strand breaks (alisb). We observed a linear increase of all types of lesions with dose and an o.e.r. for dsb of 3-6, for ssb of 4-9 and for alisb of 2-1. The data do not deviate significantly from those, measured on thymocytes irradiated without cold shock. In DNA of irradiated thymocytes, the frequency of 3' and 5' hydroxyl and 5' phosphate end-groups was analysed enzymatically. In both the ssb and alisb, about 11 per cent of the terminals carry 5'OH end-groups and 20-40 per cent 5' phosphate groups. On the 3' terminals, 60-80 per cent of the ssb are identified as 3'OH end-groups, whereas on the alisb only a small amount of 3'OH end-groups if found. The frequency of characterized end-groups shows the same oxygen effect as the corresponding strand breaks. Therefore, in the presence and absence of oxygen, the same mechanism may be responsible for formation of DNA strand breaks in vivo.

Animals↗
PubMedDate not supplied

Detachment of segments from DNA double strands as detected by time resolved Rayleigh light scattering.

Calf thymus DNA was irradiated oxygen saturated 0.01 N NaCl solution with 2 mus pulses of 15 MeV electrons. By monitoring the decrease of light scattering intensity after the pulse, two modes of decrease were detected: The fast decrease (tau 1/2 approximately 0.8 ms) is ascribed to the separation of DNA fragments produced by double strand breaks at positions directly opposite to each other. The slow decrease (tau 1/2 approximately 8 s) is attributed to the detachment of segments generated by single strand breaks at sites on the alternate strands being separated by about 10 nucleotide units.

DNA↗